How Stuff Works: How Fusion Propulsion Will Work: Shows how building a fusion-powered spacecraft would be equivalent of developing a car on Earth that can travel twice as fast as any car with a fuel efficiency of 7,000 miles per gallon.
Innovative Nuclear Space Power and Propulsion Institute: Researching square lattice honeycomb ternary carbide fuels and nuclear electric propulsion based on gaseous and liquid fuel reactors with magnetohydrodynamic power generation systems, among other things.
NERVA: The Nuclear Engines for Rocket Vehicle Applications project. Started by the Atomic Energy Commission and NASA in 1961 and cancelled in 1971 due to radiation worries and a lack of interest from NASA.
Nuclear Thermal Propulsion: Federation of American Scientists' review of the topic.
NuclearSpace: The goal is to promote the use of nuclear power in space and to further enhance the manned exploration of our solar system.